Retrieve attributes with 16bit uuid if 128bit fails.

When retrieving attribute handles using the 128bit base version of a 16 bit UUID some devices will
report "not found". This will attempt to convert the UUID to the 16bit version and try again.

* Adds `to16()` method to NimBLEUUID.

src/NimBLEUUID.h

@@ -48,6 +48,7 @@ public:
     bool                  equals(const NimBLEUUID &uuid) const;
     const ble_uuid_any_t* getNative() const;
     const NimBLEUUID &    to128();
+    const NimBLEUUID&     to16();
     std::string           toString() const;
     static NimBLEUUID     fromString(const std::string &uuid);